1. Field of the Invention
The present invention relates to a thin film transistor (hereinafter referred to simply as a "TFT") and a method for fabricating the same. In particular, the present invention relates to the structure and the fabrication process of the TFTs used as switching elements for selecting a pixel in a liquid crystal display device (hereinafter referred to simply as an "LCD") or as driver elements for driving the LCD.
2. Description of the Related Art
An active matrix substrate in an LCD is provided with a plurality of TFTs arranged in a matrix, a plurality of pixel electrodes made of an ITO (indium tin oxide) film each corresponding to one of the TFTs, and a conductive layer connected to source and drain regions. As the conductive layer including source and drain electrodes, and source and drain lines (hereinafter referred to as the "source/drain electrode lines"), an Al (aluminum) thin film is used.
The Al film is patterned using a resist so as to serve as the source/drain electrode lines in the substrate having the above-described configuration. When the resist is developed with a developing solution, however, the developing solution causes a cell reaction between the Al film and the ITO film, thereby causing corrosion of the Al film.
The corrosion can be prevented, for example, as follows. An insulating film is deposited on the source/drain electrode lines formed by patterning the Al film. A contact hole is provided through the depth of the insulating film so as to electrically connect the source/drain electrode lines to the pixel electrodes located above the electrode lines. The ITO film is then deposited over the insulating film. During the deposition of the ITO film, however, portions of the Al film exposed via the contact hole running through the insulating film is oxidized by an oxygen atmosphere in a deposition apparatus. As a result, there arises a problem that an ohmic contact cannot be obtained between the ITO film and the source/drain electrode lines.
In order to solve the above-mentioned problems, the source/drain electrode lines can be a double layer including an Al film and an Mo (molybdenum) film deposited on the Al film. In such a case, the corrosion can be prevented, while it is possible to obtain an ohmic contact between the ITO film and the source/drain electrode lines.
Moreover, in the case where the source/drain electrode lines have the Mo/Al double layer structure, where the Mo film is deposited on the Al film, it is possible to suppress the occurrence of Al bumps, i.e., hillocks or whiskers, which prevents the migration of the Al film, and thus enhances the reliability of the lines.
In the case where the source/drain electrode lines have the Mo/Al double layer structure, the two layers can be successively deposited. Moreover, the two layers can be simultaneously wet-etched using a mixed solution containing phosphoric acid, nitric acid, acetic acid, and H.sub.2 O. It is thus possible to solve problems such as corrosion without increasing the number of the processing steps.
However, since the etching rates of the Al film and the Mo film are different from each other, there arises a problem that the pattern of the Mo film shifts with respect to the pattern of the Al film as a result of the wet-etching. In other words, the line width of the pattern of the Mo film or the Al film becomes smaller than that of the other.
For example, as a result of the wet etching, a lower layer film 21 of the source/drain electrode lines may be etched more in the lateral direction than an upper layer film 22 thereof, as shown in FIG. 6A. When another film 23 (e.g., an insulating film or a protective film) is formed over such source/drain electrode lines, a cavity 24 may be generated, which may then develop to be a crack 25, in the vicinity of the interface between the lower layer 21 and the upper layer 22 of the source/drain electrode lines. In subsequent processing steps, an acid or the like may permeate into the cavity 24 or the crack 25, and cause corrosion in the source/drain electrode lines. Thus, there arises a problem such as disconnection of the lines.
Conversely, as a result of the wet etching, the upper layer film 22 of the source/drain electrode lines may be etched more in the lateral direction than the layer film 21 thereof. As a result, the line width of the pattern of the upper layer 22 becomes smaller than that of the lower layer 21, as shown in FIG. 6B. In such a case, the coverage of an ITO film 26 over a contact hole 23a provided above the source/drain electrode lines is incomplete. Accordingly, the contact resistance thereof is high, whereby it is difficult to obtain electric conductivity between the ITO film 26 and the source/drain electrode lines.
As shown in FIG. 6B, the lower layer film 21 of the source/drain electrode lines may be exposed in the contact hole 23a provided above the source/drain electrode lines. In such a case, when a mask pattern is dislocated in a mask alignment process for etching the ITO film 26, a portion 27 of the source/drain electrode lines is corroded by, for example, an etching solution for etching the ITO film 26 or other acids used in subsequent processing steps.
Also, in view of the reliability (i.e., the structural strength) of the Al film of the Mo/Al double layer structure of the source/drain electrode lines, when the etching shift of the Al film lower layer 21 is large, arises a problem that the coverage of the film covering the source/drain electrode lines is likely to be deteriorated. On the other hand, when the etching shift of the Mo film upper layer 22 of the Mo/Al double layer structure is large, there arises a problem that the anti-hillock property or the like, in a heat-treatment for the Al film, is deteriorated.
Japanese Laid-Open Patent Publication No. 6-104241 discloses a method for etching a double layer structure where a thickness ratio of layered films is controlled to reduce the amount of side etching. Based on experiments conducted by the inventors of the present invention, it is confirmed that, in the case of the Mo/Al double layer structure, the line width of the Mo film becomes smaller than that of the Al film regardless of the thickness ratio thereof.